As to a speech recognition installed into a car-navigation system, a barge-in function to recognize a user's speech during reproducing a system sound (For example, a beep sound or a guidance speech) is prepared. As to a speech discrimination used for preprocessing of the speech recognition, it is required that the user's speech is correctly extracted from an acoustic signal including a reproduced sound (echo) as the system sound reproduced from a speaker. For example, as the speech discrimination to raise robustness for the system sound, a following method is proposed. In this method, a frequency band including a main power of the system sound is specified, and, when a feature is extracted from the acoustic signal, a frequency spectrum in the frequency band is excluded. As to the frequency spectrum in the frequency band including the main power of the system sound, a probability that an influence of the echo is included therein is high. Accordingly, by excluding the frequency spectrum from the frequency band, the feature excluding the influence of the echo (caused by the system sound) can be extracted.
However, in above-mentioned method, a case that the system sound is an acoustic signal of a single channel is imagined. Accordingly, in a case that the system sound is an acoustic signal of a plurality of channels such as a stereo music, this method cannot cope with.